Electric heaters



Aug. 5, 1958 Filed larch 3. 1954 N. B. THORNHILL ELECTRIC HEATERS 2Shuts-Sheet 1 JNVENTOR. New B. Taoaumu.

RITORNEY Aug. 5, 1958 N. B. THORNHILL ELECTRIC HEATERS 2 Sheds-Sheet 2Filed March a, 1954 uvmvroa. NORMAN B. 'Ifionumu.

Hnoauzv United States Patent ELECTRIC HEATERS Norman B. Thornhill,Wilmerding, Pa., assignor to Edwin L. Wiegand Company, Pittsburgh, Pa.,a corporation of Pennsylvania Application March 3, 1954, Serial No.413,920

3 Claims. (Cl. 201-67) The present invention relates to methods ofmaking sheathed electric resistance heaters and to the article of.

manufacture produced thereby, and the principal object of my inventionis to provide new and improved methods and articles of this character.

In making sheathed electric resistance heaters of the type having anelectric resistor disposed within a deform-- able sheath which is filledwith electric-insulating, heatconductive granular refractory material,it is necessary, after the filling operation, to close the ends of thesheath by some means to prevent loss of the refractory material duringsubsequent operations.

In many constructions wherein annealing of the heater sheath is anecessary part of the manufacturing operation or wherein the ultimateuse of the heater is such that the terminal ends thereof will besubjected to elevated temperatures, considerable difliculty has attendeddevelopment of a suitable closure for the sheath.

In the past, the ends of the sheath have been closed by an operationknown in the trade as end-packing and such operation comprised placingone or more pellets formed of a brittle dielectric material in arespective open end of the sheath and crushing such pellets by repeatedblows delivered through such open end until a tightly packed wad ofdielectric material had been formed. Following end-packing each end ofthe sheath, the portion of the sheath intermediate the ends wascompressed as by being pressed between dies in a press or the like so asto partially compact the refractory material about the resistor. Afterpartially compacting the refractory about the resistor, it was possibleto form the sheath, by bending or the like, to the desirable shape afterwhich the previously mentioned intermediate portion of the sheath wasfurther pressed as between dies or the like, to further compact therefractory material about the resistor.

The above described prior art method of making sheathed electric heatershas been more or less satisfactory; however, this method has been quiteexpensive and a considerable portion of this expense has been theendpacking operation. Accordingly, it is an object of my invention tosimplify and reduce the cost of producing electric heaters of thecharacter described. The means by which this and other objects areachieved will become apparent from a study of the following descriptionand from the drawings appended hereto.

In the drawings accompanying this specification and forming a part ofthis application there is shown, for purpose of illustration, twoembodiments which my invention may assume, and in these drawings:

Figure l is a broken perspective view of a sheathed electric heaterconstructed in accordance with my invention,

Figure 2 is a perspective view of a detail,

Figure 3 is a broken, longitudinal sectional view of an electric heaterillustrating one step in the manufacture thereof,

Figure 4 is a view similar to Figure 3 but illustrating the heater afteranother step,

Figure 5 is a broken, elevational view of the electric heaterillustrating its appearance following still another step,

Figure 6 is a sectional view generally corresponding to the line 66 ofFigure 5, Figure 7 is a fragmentary view of the heater illustrated inFigure l but somewhat enlarged and shown in longitudinal section,

Figure 8 is a sectional view generally corresponding to the line 8-8 ofFigure 7,

Figure 9 is a plan view of another type heater constructed in accordancewith my invention,

Figure 10 is a fragmentary view generallycorresponding tothe line 10-10of Figure 9, and

Figure 11 is a broken fragmentary elevational view illustrating a stepin the formation of the heater shown in Figure 9.

In carrying out my invention of providing sheathed electric heaters 10of the type shown in Figure 1, I preferably employ the conventionalsteps of positioning a coiled resistor 11 having a current-carryingterminal pin 12 conventionally secured to each end thereof within around, deformable hollow sheath 13 whereby each terminal pin 12 extendsbeyond a respective end of the sheath 13 as shown (see Figure 3). Thesheath 13 is adapted to be filled with any well-knownelectric-insulating, heat-conducting refractory material 14 which is, inthe presently disclosed embodiment, granular in form. Any suitable meansmay be employed to fill the sheath 13; for example, any suitablecommercially available filling machine may be utilized and since suchmachines are well-known in the art, a detailed description thereof isbelieved to be unnecessary.

During the filling operation as herein disclosed, the sheath 13 isadapted to be positioned generally vertically and the refractorymaterial 14 is introduced into the uppermost end of the sheath andallowed to gravitate down to fill the sheath. At the present time it ispreferable to close the bottom of the sheath by means of a plug 15 (seeFigure 2) which is of a size to closely fit within the sheath 13 andwhich is apertured to pass the lower terminal pin 12 with a minimum ofclearance.

The plug 15 may be formed of any suitable dielectric material which isdeformable without excessive crumbling and which is capable ofwithstanding elevated temperatures. I have found that a plug machinedfrom a finegrained igneous mineral substance commonly known as lava iswell-suited for the purpose since its density is high and it is easilyhandled yet is sufiiciently deformable to withstand a limited amount ofcompression without crumbling; it possesses satisfactory dielectricqualities;

Another type of plug found suitable for use in the present invention isone formed of compacted, powdered aluminum oxide held together by anysuitable bonding agent. This type of plug possesses at least a minimumof the desirable characteristics of the above described lava plug andadditionally is considerably lower in cost since it may be made by anextrusion process instead of being machined.

. After the sheath 13 has been filled with refractory material, a plug16, similar in all respects to plug 15, may be positioned in the upperend of the sheath (see Figure 4). It will be appreciated that the sheathwill not be completely filled with the refractory material but thatsufficient space will be provided for receiving the plug 16.

With the sheath filled with refractory material and the plugs 15, 16 inposition in the ends of the sheath, the sheath ends will next becompressed laterally to secure 3. the plugs tightly in position toprevent loss of refractory material through such ends. Since as aprerequislte to subsequent operations it is usually necessary topartially compact the refractory material 14 about the resistor 11, itis preferable at the present time to compress the ent1re length of thesheath so as to both secure the plugs tightly in position and partiallycompact the refractory material in a single operation.

At the present time, it is preferred that the sheath be compressed to ahexagonal configuration (see Figures 5 and 6); however, it will beappreciated that the sheath could be compressed to any other suitableconfiguration. Any desirable means may be employed to compress thesheath inthe manner disclosed; for example, the sheath maybc'c'o'mpressed between suitable dies carried by a press or the like orfed between suitably arranged rolls which will accomplish the desiredresult.

After the heater has been formed as shown in Figures 5 and 6 anddepending upon the nature of the material from which the sheath is made,it is customary to subject the heater to a temperature of at least 1950degrees Fahrenheit for approximately fifteen minutes in order to annealthe sheath for further operations.

In some instances when lateral compression is applied to the sheath tosecure the plugs and at least partially compact the refractory material,it has been found that the lava plugs have been crushed to an extentwherein voids may appear therein and the annealing heat hereinbcforementioned accomplishes the further function of maturing or at leastpartially vitrifying the lava to close any voids. Further, maturing orvitrification increases the strength of the lava plug and therebyoff-sets any loss of strength which may be caused by the crushing whichtook place when the lava bushings were locked in place by the compactingforce.

As will later be described, further operations may include bending thesheath to form a convoluted or otherwise shaped heater unit; however, informing the straight heater illustrated in Figure l, and following theanneal ing operation (if employed), the portion of the sheathintermediate the ends thereof will be further compressed to furthercompact the refractory material 14 about the resistor 11 (see Figures 1,7 and 8). Any suitable means may be employed to compress this portion ofthe sheath without disturbing the ends thereof where the bushings 15, 16are disposed. For example, the sheath may be placed between the dies ofa press, the dies being proportioned to exert pressure upon only theintermediate portion of the sheath.

As herein disclosed and with respect to the heater shown in Figure l,the intermediate portion of the sheath 13 is compressed by action of apress or the like to a round configuration (see especially Figure 8);however, it is to be understood that the intermediate portion of thesheath may be compressed to any other suitable configuration, forexample, to a triangular or heart-shaped configuration if desired.

In forming the heater shown in Figure 9, the steps illustrated inFigures 3, 4 and 5 will be employed.

Following the step shown in Figure 5, the heater will be annealed, ifnecessary, as before described. After annealing, if this operation isused, portions 17 of the heater (see Figure 11) will, as hereindisclosed, preferably be compressed to a round cross-section in mannersimilar to that employed in forming the heater shown in Figure 1.However, as illustrated, onlythe portiohs 17 near the ends of the heaterwill be so compressed, the extreme ends 18 of the heater and the portion19 intermediate the portions 17 will not be compressed.

After the heater has been formed as shown in Figure 11, the portion 19of the heater will be formed to a spiral configuration (see Figure 9)and the portions 17 will be bent to the spiral as shown.

Following the operation of forming the heater to the spiralconfiguration disclosed, the spiraled portion 19 of the heater will becompressed to further compact the refractory material about theresistor. At the present time, the portion 19 is placed between dies andcompressed to a triangular or heart-shaped configuration (see Figure10); however, it will be appreciated that the portion 19 could becompressed to a round, oval or any other desired shape.

Under certain circumstances it may be desirable to eliminate theoperation of compressing the portions 17 of the sheath and to coil theportion 19 of the heater after it has been formed as shown in Figure 5.

While I have illustrated and described the formation of only two typesof heaters, a straight heater and a convoluted or spiral heater, it willbe appreciated that any other suitably shaped heater may be formed ifdesired.

In view of the foregoing it will be apparent to those skilled in the artthat I have accomplished at least the principal object of my inventionand it will also be apparent to those skilled in the art that theembodiments herein described may be variously changed and modified,without departing from the spirit of the invention, and that theinvention is capable of uses and has advantages not herein specificallydescribed, hence it will be appreciated that the herein disclosedembodiments are illustrative only, and that my invention is not limitedthereto.

I claim:

1. The method of producing a sheathed embedded resistor electric heaterwherein a resistance conductor is disposed within a metal tubular sheathand has a terminal end accessible through an opening in an end of saidsheath, and wherein the resistance conductor is surrounded by andembedded in electric-insulating heatconducting refractory material, thelatter stopping short of said tubular opening to form therewith apocket, which method comprises disposing an apertured lava plug in saidpocketin encompassing relation with respect to the accessible end ofsaid conductor, compressing and deforming the sheath radially inward tocompress it about said plug and force the latter into intimateengagement with the entire inner wall of said pocket and with theadjoining peripheral surface of said conductor, and thereaftersubjecting at least said sheath end to a temperature sufiicient tovitrify said lava plug.

2. An electric heating element, comprising a tubular metal sheath, aresistance conductor member having a terminal end extending through anend opening of said tubular sheath, a refractory heat conductingmaterial within said sheath for electrically insulating said conductormember from said sheath, said refractory material terminating short ofsaid sheath opening to form therewith an open pocket in said sheath, andan apertured lava plug held in tight engagement with the entire pocketinner wall and the adjoining exterior surface of said conductor terminalby a transverse deformation of said sheath at its end opening.

provide terminal ends which extend from 3. The construction according toclaim 2 wherein said lava plug is vitrified.

Great Britain Dec. 9, 1929

